Establishing a group call in a mobile radio system

ABSTRACT

A method and mobile radio system for establishing a group call extending to service areas of several exchanges in a mobile radio system which includes at least a first exchange and a second exchange, telecommunication media for establishing communication connections between at least the first exchange and the second exchange, and a plurality of mobile stations communicating via the exchanges. The method includes selecting a second exchange to which the group call is routed from the first exchange. To save telecommunication resources in a mobile radio system, checking, before routing the call to the second exchange, whether a telecommunication medium has been previously allocated on the connection from the first to the second exchange for the group call to be established, and, if so, routing the group call to be established from the first to the second exchange without allocating a new telecommunication medium to that connection.

FIELD OF THE INVENTION

This application claims benefit of international applicationPCT/FI95/00614, filed Nov. 10, 1995.

The invention relates to a method for establishing a group callextending to service areas of a plurality of exchanges in a mobile radiosystem comprising at least a first and a second exchange,telecommunication media for establishing communication connectionsbetween at least the first and the second exchange, and mobile stationscommunicating via said exchanges; the method comprising the step ofselecting a second exchange to which the group call is routed from thefirst exchange.

The invention relates to a mobile radio system in which group calls areto be established between subscribers that may be located in areas of aplurality of exchanges.

BACKGROUND OF THE INVENTION

The invention is intended for use especially in so-called trunkingnetworks that are typically company networks or private mobile radionetworks used by authorities, all the channels of such networks beingused by one or more companies or authority organizations. In addition tosubscriber numbers, the subscribers of these networks have group numbersindicating the group call group or subscriber group that the subscriberbelongs to. This makes it possible to transmit calls addressed to themembers of a certain group to the subscribers of the group.

The invention is suitable for use in mobile radio systems having eithera digital or an analog radio paths. The digital mobile radio systems maycomply, e.g., to the TETRA system (Trans European Trunked Radio System).Analog mobile radio systems are described, e.g. in MPT 1327, ASignalling Standard for Trunked Private Land Mobile Radio Systems,January 1988, revised and reprinted November 1991, and MPT 1343,Performance Specification, January 1988, revised and reprinted September1991, both issued by the Radiocommunications Agency, published by theBritish Department of Trade and Industry.

One essential feature of, e.g., the above mentioned mobile radio systemsis that they allow and are designed for the implementation of a groupcall between several subscribers.

A group call is a conference call during which all the participants areable to both speak and listen to one another, in turns. In group calls,the entire group is called by a single call number. An individual mobilestation (e.g. radio phone), or a subscriber station, may belong toseveral groups programmed in the respective mobile station. The systemcontains a file storing information about base stations associated withthe number of each group. A group call may cover one, more one of or allbase stations located within the area of a mobile exchange, or aplurality of mobile exchanges. To establish a group call, each basestation belonging to the group allocates a traffic channel and transmitsa group call request containing a group number and information about thetraffic channel allocated. If a mobile station identifies as being oneof a group to which it belongs the group number contained in the groupcall request, it switches to the traffic channel indicated by the groupcall request. In principle, a mobile station is thus always able toenter a group call, if it is located within the predetermined operationarea of the group and if the request to enter the call originates inthat area.

A group call is usually a characteristic of networks designed forspecial purposes. Such networks do usually not comprise many exchanges.Conventionally, the area for setting up a group call has been limited,e.g., to the area of one exchange. The problem of establishing a groupcall in a network of several free-connected exchanges has thus arisen asmobile radio systems have become larger in size and as one has startedto establish group calls extending to service areas of severalexchanges.

The conventional way of establishing a group call to service areas ofseveral exchanges is to try and route the group call as a whole and notbranch it until as close to the destination exchanges as possible.

The known methods for establishing a group call do not offer a routingtechnique to a free-connected network comprising several exchanges, tothe service areas of which a group call is to be established. Themethods do not take into account situations where a call must be routedvia several exchanges and branched at suitable points. In such cases,the major problem is to find optimal routes and minimize the use ofconnections and resources.

FIG. 1 shows a mobile radio system of the prior art, illustratingproblems involved therein. The figure shows exchanges 10 to 16. To theexchanges are connected base stations, to which in turn are connectedmobile stations via a radio path, e.g. an A-subscriber A-SUBSCRIBER,which sends a group call set-up request, and a B-subscriberB-SUBSCRIBER, which is invited to enter the call in addition to theA-subscriber. In the prior art, when a group call is established toservice areas of several exchanges, a first exchange 10, whichestablishes the group call to the service areas of other exchanges 12,14, 15, 16 by the command of the A-subscriber A-SUBSCRIBER, firstestablishes the call to the second exchange 12 and then to the thirdexchange 15. The first exchange then establishes the call to the fourthexchange 14 and the fifth exchange 16. In the prior art, the firstexchange 10 establishing the call first establishes the call to thesecond exchange 12 and allocates necessary telecommunication resources101, such as a telecommunication medium, between the respectiveexchanges. The first exchange 10 establishing the call then establishesthe call to the third exchange 15, e.g. via the second exchange 12 andallocates communication connections for the call from the first exchange10 to the third exchange 15 via the second exchange 12. One problem hereis that telecommunication resources may be allocated twice between thefirst 10 and second exchange 12 for the group call to be established:first when a call is established, to mobile stations located in theservice area of the second exchange 12, and second, to the mobilestations located in the service area of the third exchange 15, or toother exchanges 16. In FIG. 1, the thickness of the lines betweenexchanges 10, 12, 14, 15 and 16 describing telecommunication buses,resources and media indicates how many telecommunication media areallocated for one and the same group call on the connection concerned.Another problem with a solution like this is that it slows down callset-up, since at least double resources have to be allocated for aspecified call, and so time is wasted on the allocation of `surplus`resources. Yet another problem is that call set-up may even be hinderedif no free telecommunication resource is available between the first 10and second 12 exchange for a group call to be established to the thirdexchange 15.

In a star-like network, a group call is easy to establish and the aboveproblem does not arise, since the nodal exchange establishing the groupcall routes the call to exchanges that are associated with subscribersbelonging to the group. The nodal exchange can then allocate only onespeech connection or other telecommunication resource or medium to eachexchange requiring such a connection. A star-like network, however, isnot suitable for all mobile radio systems, especially not for extensivemodern systems.

SUMMARY OF THE INVENTION

One object of the invention is to route a group call in a sensiblemanner to several subscribers located in service areas of severalexchanges such that optimal routes and minimal resources are used.

Another object of the invention is to solve the problems involved in theprior art. In particular, the invention aims at eliminating the problemcaused by the fact that when a group call is established to serviceareas of several exchanges, double or multiple telecommunicationresources, such as communication connections or other telecommunicationmedia, may be unnecessarily allocated for one and the same group callbetween two exchanges.

Yet another object of the invention is to speed and secure group callset-up in a situation where a group call is established in service areasof several exchanges.

A further object is to simplify call set-up in a situation where a groupcall is to be established to subscribers located in service areas ofseveral exchanges.

This new type of method for establishing a group call extending toservice areas of several exchanges is achieved with the method of theinvention, which is characterized by comprising the steps of checking,before the call is routed to the second exchange, whether atelecommunication medium has been previously allocated on the connectionfrom the first to the second exchange for the group call to beestablished; routing the group call to be established from the first tothe second exchange without allocating a new telecommunication medium tothe connection if a telecommunication medium has been previouslyallocated on that connection for the group call to be established.

The invention also relates to a mobile radio system for establishing agroup call extending to service areas of several exchanges, the mobileradio system comprising at least a first and a second exchange,telecommunication media for establishing communication connectionsbetween at least the first and the second exchange, and mobile stationscommunicating via the exchanges.

The mobile station system according to the invention is characterized bycomprising check means for checking whether a telecommunication mediumhas been previously allocated on the connection between the first andthe second exchange for the call to be established.

The basic idea of the invention is that before a call is routed from thefirst, call establishing, exchange to the second exchange, one checkswhether a telecommunication medium has been previously allocated on theconnection from the first to the second exchange for the group call tobe established. The group call to be established is then routed from thefirst to the second exchange without allocating a new telecommunicationmedium to the connection if a telecommunication medium has beenpreviously allocated on that connection for the group call to beestablished.

The invention is thus based on a check carried out in allocatingresources to connections between exchanges to see whether resources havebeen previously allocated for the call on the connection concerned. Theobject of the invention is to make individual call branches, i.e.connections between exchanges, use common resources. This isimplemented, e.g., such that the call set-up process records in thedatabase the resources allocated for the call on each connection,thereby preventing allocation of multiple resources on one connection.When the call is forwarded from the exchange, it is possible to checkfrom the database that no telecommunication medium has been previouslyallocated to the connection.

One advantage of such a method for establishing a group call extendingto service areas of several exchanges in a mobile radio system, and of amobile radio system is that they solve the problems involved in theprior art. A particular advantage of the invention is that it eliminatesthe problem of unnecessarily allocating double or multipletelecommunication resources, such as communication connections or othertelecommunication media, for one and the same call between two exchangeswhen a group call is established to service areas of several exchanges.

Another advantage of the invention is that it also speeds and securesgroup call set-up when a group call is established to service areas ofseveral exchanges.

Yet another advantage of the invention is that it simplifies call set-upin a situation where a group call is to be established to subscriberslocated on service areas of several exchanges.

A further advantage is that the solution provided by the invention doesnot place restrictions on the routing method used. When a call is routedto a single exchange, the invention makes it possible to use differentmethods that are considered the most suitable for the network andapplication concerned. This solution makes it possible to set up groupcalls in rather different networks operating in different manners.Different resource and load allocation functions can thereby beperformed by the actual routing method, and the present solutionprovides an effective method for establishing group calls.

A still further advantage is that in the implementation of theinvention, it is not very significant to the routing which exchange isselected to establish the call. The method functions rather well,independently of the exchange associated with the group call that isselected to start the call set-up.

A further advantage of the invention is that subscribers can enter anon-going group call. The solution of the invention makes it possible toengage a new subscriber in a group call without any exceptional callset-up procedures.

Another advantage of the invention is that it enables effective use ofthe resources of a mobile radio system without complicated andtime-consuming deduction algorithms for selecting the route and suitablepoints of branching.

Yet another advantage of the invention is that it makes the use ofcomputing capacity and connection resources of the mobile radio system,especially those of its exchanges, more effective in group call set-up.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail withreference to the attached drawings, in which:

FIG. 1 is a block diagram of a mobile radio system according to theprior art, illustrating problems involved therein, and

FIG. 2 is a block diagram of a mobile radio system according to theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 is described above in connection with the description of thebackground art.

FIG. 2 is a block diagram of a mobile radio system according to theinvention, the system comprising exchanges 10 to 16. In a network set upfor practicing the invention, establishment of a group call is startedby checking what exchanges are associated with the group call. Then, theexchange or exchanges for establishing the call must be selected. Thesimplest solution is to select one exchange that establishes a call toall other exchanges associated with the group call. The exchange may be,e.g., an exchange in which is stored information about the groupconcerned, or it may be an exchange selected at random.

The exchange establishing the call establishes the connections betweenthe exchanges involved in the call separately to each exchange. When theexchange establishing the call or any exchange on the route selects aroute forward, it checks, according to the invention, whether it can usethe resources previously allocated for the group call on the routeconcerned. Such resources include a telecommunication medium or atelecommunication resource, such as a telecommunication bus, or atelecommunication channel, a telecommunication time slot, or, e.g.,multiplexing, encoding or decoding means. If resources have beenpreviously allocated on the connection concerned for the group call tobe established, the exchange instructs the call to use the sameresources.

FIG. 2 shows a simple example for a mobile radio network.

If a group call is to be established in the network of FIG. 2 includingexchanges 10, 12, 13, 14, 15 and 16, exchange 10 can be selected forcall establishment. The exchange then establishes connections 10--12,10--13, 10--14, 10--15 and 10--16 between the exchanges separately.Individual branches can be assumed to be routed e.g. as follows:

branch A: 10--12: 10-12

branch B: 10--13: 10-13

branch C: 10--14: 10-12-14

branch D: 10--15: 10-12-15

branch E: 10--16: 10-12-15-16.

If the call branches are established in the above order, exchange 10notices upon establishing branches C, D and E that a connection has beenpreviously allocated for the call on connection 10--12, so no newresources need to be allocated. Upon establishing branch E, exchange 12notices that a connection has been previously allocated and thus no newconnection needs to be allocated. Thus, only one speech connection needsto be allocated for the entire call on connections 10--12, 10--13,12--14, 12--15 and 15--16.

The optimal implementation depends on the routing method used, the groupcall type, and the structure of the network. In the most generallyapplicable and versatile solution, a group call is established by oneexchange in the group call area, and joint use of resources is based onthe use of an identity specific for each group call.

With regard to the load caused by the call set-up, it is preferable thatonly one exchange, which itself belongs to the group call area,establishes the call. Depending on the routing method, this is often thebest routing solution.

The use of identities specific for each group call is advisable, sinceit makes it possible to guide subscribers entering an on-going call touse resources previously allocated for the call concerned.

In the invention, a group call is established as follows:

1. A group call is divided into call branches destined to differentexchanges. All call branches are given the same unambiguous group callidentity CALL ID specific for each call.

2. One exchange starts to route the different call branches separatelyto the destination exchanges.

3. The exchange performing the routing determines, on the basis of adesired routing algorithm, the exchange to which the call branch is tobe routed next. Together with call set-up requests, information aboutthe call identity CALL ID is transmitted.

4. When the next exchange of the route has been selected for a callbranch, a request is transmitted to the system that allocates resources,e.g. to a software process, to allocate resources to said exchange.

5. The resource manager checks the database to find out whetherresources have been previously allocated to said exchange by the samecall identity CALL ID. If resources have been allocated, the call set-upunit of the exchange concerned is informed of the resources available tothe call branch, and the call set-up process can proceed to the nextexchange of the route. If no resources have been allocated before, theyare allocated at this stage, and the call set-up unit is informed of theresources available to the branch, and the process continues as above.

6. The routing of a call branch is continued until the branch hasreached the destination exchange.

In the following, the structure of the mobile radio system of theinvention is described with reference to FIG. 2.

To establish a desired group call, the mobile radio system of theinvention comprises check means 201 for checking whether atelecommunication medium has been previously allocated for the groupcall concerned on the connection from the first 10 to the second 12exchange.

The mobile radio system of the invention further comprises a database202 which indicates, specifically for each group call, thetelecommunication media 101 that are allocated for the respective groupcall on each connection from the first 10 to the second 12 exchange.

In the mobile radio system of the invention, the check means 201 arearranged to allocate a telecommunication medium 101 on the connection(10--12) for the group call to be established, if said database 202indicates that no telecommunication medium 101 has been previouslyallocated for the group call on the connection. Further, the check means201 are arranged to route the group call to be established from thefirst to the second 12 exchange by the allocated telecommunicationmedium 101.

Further, in the mobile radio system of the invention the check means 201are arranged to route the group call to be established from the first 10to the second 12 exchange with the telecommunication medium 101previously allocated for the group call concerned if the database 202indicates that a telecommunication medium 101 has been previouslyallocated on the connection from the first 10 to the second 12 exchangefor the group call to be established.

The mobile radio system of the invention further comprises means 203 formarking the allocation of the telecommunication medium 101 in thedatabase 202, if the database 202 does not indicate that atelecommunication medium 101 has been previously allocated for the groupcall on the connection.

Thus, the basic idea of the invention is that several connections areestablished between different exchanges for the same group call in amobile radio system such that only one telecommunication resource ormedium, such as a telecommunication channel or time slot, is allocatedbetween two exchanges for one and the same group call. The connectionsestablished between two exchanges for one and the same group call thususe the same telecommunication resource.

The exchange establishing the call can be selected and the resources forone and the same call combined quite freely. The exchange establishingthe call may be the exchange of an A-subscriber or an exchange locatedin the service area of the group, or several exchanges may be involvedin the call set-up. In the latter case, e.g. the exchanges of allB-subscribers (and C-subscriber) may establish a call to the exchange ofthe A-subscriber.

The resources used can be combined by many different methods. Forexample, each call or each group may have its own identity that can beused in the combination process. The method may also be morecomplicated: e.g. it is possible to send a message on a group callconnection, whereby the exchange knows on the basis of the message howto establish the connections on which the message is transmitted.

The drawings and the description thereof are to be understood asillustrating the idea of the invention. The method and mobile radiosystem according to the invention may vary in their details within thescope of the claims. Although the invention is described above mainlywith reference to radio phone systems, it can also be used in otherkinds of mobile radio systems.

I claim:
 1. A method for establishing a group call extending into therespective service areas of a plurality of exchanges in a mobile radiosystem which includes at least a first and a second exchange, atelecommunication transmission medium between said first and secondexchanges, and a plurality of mobile stations communicating via saidexchanges, said method comprising the steps of:starting to establish agroup call involving a plurality of said mobile stations, located withinthe respective service areas of a plurality of said exchanges, includingsaid first and the second exchanges, selecting said second exchange tobe one to which the group call is to be routed from said first exchange,maintaining a database which indicates, specifically for each groupcall, the telecommunication transmission medium that is allocated forthat group call on each connection between respective ones of saidexchanges, checking said database before routing said group callstarting to be established from said first to said second exchange, tosee whether a telecommunication transmission medium has been previouslyallocated on the respective said connection from said first to saidsecond exchange for said group call starting to be established, routingsaid group call starting to be established from said first to saidsecond exchange using a respective previously allocatedtelecommunication medium, without allocating a new telecommunicationtransmission medium to the respective said connection, if atelecommunication transmission medium has been previously allocated onthe respective said connection for said group call starting to beestablished, as determined from said checking.
 2. The method accordingto claim 1, further comprising the steps of:if said database, when saidchecking is performed, does not indicate that a telecommunicationtransmission medium has been allocated for said group call starting tobe established on the respective said connection, allocating atelecommunication transmission medium for said group call about to beestablished on the respective said connection between said first andsecond exchanges, marking allocation of the respective saidtelecommunication transmission medium in said database, and routing saidgroup call starting to be established, from said first to said secondexchange with the respective said telecommunication transmission mediumallocated in said allocating step.
 3. A mobile radio system forestablishing a group call extending into the respective service areas ofa plurality of exchanges, comprising:a plurality of exchanges, includinga first and a second exchange each having a respective source area, aplurality of mobile stations communicating via said exchanges, includingvia said first and second exchanges, a telecommunication transmissionmedium for establishing a group call between said first and secondexchanges, a database which indicates, for each group call, thetelecommunication transmission medium that is allocated for therespective said group call on each connection from said first to saidsecond exchange, checking means for checking whether a telecommunicationtransmission medium has been previously allocated on the respective saidconnection from said first to said second exchange for said group callto be established, said checking means being arranged to:(a) allocate atelecommunication transmission medium on said connection between saidfirst and said second exchange for said group call to be established, ifsaid database indicates that no telecommunication transmission mediumhas been previously allocated for said group call on said connectionbetween said first and said second exchange, and (b) route said groupcall to be established from the first to the second exchangerespectively via said allocated or previously allocatedtelecommunication transmission medium.
 4. A mobile radio system forestablishing a group call extending into the respective service areas ofa plurality of exchanges, comprising:a plurality of exchanges, includinga first and a second exchange each having a respective source area, aplurality of mobile stations communicating via said exchanges, includingvia said first and second exchanges, a telecommunication transmissionmedium for establishing a group call between said first and secondexchanges, a database which indicates, for each group call, thetelecommunication transmission medium that is allocated for therespective said group call on each connection from said first to saidsecond exchange, checking means for checking whether a telecommunicationtransmission medium has been previously allocated on the respective saidconnection from said first to said second exchange for said group callto be established, means for marking allocation of the telecommunicationtransmission medium between said first and said second exchange in saiddatabase, if said database, upon checking of said check means does notindicate that the telecommunication transmission medium has beenpreviously allocated for said group call on said connection between saidfirst and second exchanges.